Existing unmanned aerial vehicles (UAVs) have been successfully tested and utilized for outdoor aerial reconnaissance missions. Experiments have shown on numerous occasions, including operations such as Desert Storm, that UAVs often go unnoticed when flying at several thousand feet over their targets. The state of the art in optics is such that high resolution video and infrared images can be gathered unobtrusively from existing platforms.
There is a similar need for UAVs that are capable of performing indoor reconnaissance missions. Of necessity, a UAV capable of performing such missions must be small enough and slow enough to fly through constricted building interiors. The requirements for a UAV to successfully perform indoor reconnaissance may include such capabilities as rapidly negotiating hallways, crawling under doors, or navigating ventilation systems in an attempt to quickly penetrate deep into a building to complete the mission. Micro air vehicles (MAVs) have been proposed to fulfill such mission requirements. MAVs are generally thought of as miniature flying machines having no dimension greater than approximately 15 cm.
Typically, MAV efforts and proposals have been directed toward fixed wing vehicles. Fixed wing aircraft generally use their forward speed to generate the lift necessary to sustain flight. However, high speed is not conducive to indoor operations because it results in reduced reaction time, especially when autonomously navigating through unknown corridors or amid obstacles. To achieve slow speed flight, fixed wing aircraft require either large wings or a method for creating circulation over the wings in the absence of fuselage translation. Fixed wing aircraft are also generally incapable of vertical takeoffs and landings.
If the wings are enlarged to decrease wing loading to accommodate slower flight, the vehicle soon loses its distinction as a "micro" air vehicle and would be ill suited for indoor operations. If a fixed wing aircraft is maintained at the scale defined for a MAV, the forward speed required for the fixed wing vehicle to stay aloft efficiently violates the criteria for negotiating constricted spaces.
There are methods for creating circulation over the wings with little or no fuselage translation. This can be done by "blowing" the surfaces of the wings to increase lift in an intelligent manner by using an internally-generated pressure source. This has been demonstrated in manned aircraft and certain experimental unmanned vehicles, but is typically inefficient unless there is a source of gas pressure already available (such as bleed air from a gas turbine engine).
Another way to move air over a wing with little or no fuselage translation is to move the wing relative to the fuselage and the surrounding air. This can be a circular motion as in a helicopter rotor. While rotary wing flying MAVs are superior to fixed wing solutions in that they may takeoff and land vertically, they tend to be mechanically complex and energy inefficient.
A rotor is mechanically simple to spin, but does not use all parts of the wing (rotor) with the same efficiency since the inner section near the rotor hub moves more slowly than the tip. As the diameter of a rotor system decreases with the size of the air vehicle design, it becomes less efficient since the velocity at the tips decreases for a given rotational frequency while the useless center portion becomes a larger percentage of the entire rotor disk. To compensate for this, the designer will tend to increase the rotation frequency of the rotor to maintain lift for a given fuselage mass and power source. The increased rotation frequency will not only increase stress in the rotating components, it will increase the frequency and energy content of the sound produced, which will tend to make the craft too noisy for covert indoor reconnaissance.
Fixed wing and rotary wing flying MAVs are likewise not suited for movement through small openings such as partially-opened doors or under closed doors. Similar problems exist for small openings like windows, air vents, and pipes.
Therefore, there is a need for a MAV that is conducive to quiet operation, slow flight, hovering, and vertical takeoff and landing. There is likewise a need for a multimodal MAV having the additional capability of ground locomotion.